[S-IT16] Structure and Dynamics of Earth and Planetary Mantles

[SIT16-06] Simultaneous measurements of thermal conductivity and thermal diffusivity of bridgmanite and post-spinel: implications of mantle dynamics and evolution

*Youyue Zhang¹, Takashi Yoshino¹, Masahiro Osako² (1.Institute for Planetary Materials, Okayama University, 2.National Museum of Nature and Science)

Keywords:Thermal conductivity, Bridgmanite, Post-spinel, High pressure, Mantle

Thermal properties of lower mantle materials govern the heat flux from the core and controls heat transfer of the largest region of the Earth. Heat flux and its lateral variations, dominates dynamics on both sides of the core mantle boundary (CMB), altering the core dynamo, the mantle convective cycle, and plate tectonics at the surface [1][2]. Knowledge of thermal conductivity of bridgmanite, known as the most abundant mineral in the Earth’s lower mantle, thus is significant for understanding the mantle convection and the evolutions of the core and the mantle. The composition and spatial distribution of bridgmanite are heterogenous in the lower mantle. The Fe and Al content in bridgmanite changes with depth [3]. Bridgmanite-enriched domains might exist in the lower mantle [4], while at 660 km ringwoodite will break into polycrystalline aggregate of bridgmanite and ferropericlase, which is called post-spinel phase [5]. The impurity effect and spatial distribution difference may influence the heat transport performance of bridgmanite. Therefore, thermal conductivity of Al, Fe-bearing bridgmanite and post-spinel should be well constrained. Several laboratory studies have tried to investigate the thermal conductivity of bridgmanite [6][7][8][9][10]. However, their experimental methods don’t allow measuring thermal conductivity directly. Also, temperature dependence of bridgmanite was only obtained by Manthilake et al., (2011) [6]. Therefore, direct and systematic measurements of thermal conductivity of bridgmanite are necessary.



In this study, thermal conductivity and thermal diffusivity of bridgmanite and post-spinel were determined simultaneously by combining multi-anvil high pressure experimental technique and pulse heating method up to 24 GPa and 1000 K. The pressure and temperature dependences of thermal conductivity of Al-bearing bridgmanite and post-spinel were obtained. The experiment results show that Al-bearing bridgmanite has higher pressure- and temperature- dependence and absolute value of thermal conductivity than post-spinel, and thermal conductivity of post-spinel has very different performance with the variation of pressure and temperature compared with previous measured ferropericlase (Fp₂₀). It may suggest that the spatial distribution of bridgmanite and ferropericlase can influence the heat transfer in the lower mantle and a chemically heterogenous CMB will lead to lateral CMB heat flux variation.





Reference: [1] Ammann et al., Earth Planet. Sci. Lett. 390: 175-185 (2014). [2] Lay et al., Nat. Geosci., 1, pp. 25-32 (2008). [3] Irifune et al., Science 327, 193–195 (2010). [4] Ballmer et al., Nat. Geosci., 10(3), 236–240 (2017). [5] Ishii et al., Nature Geoscience, 12(10), 869–872 (2019). [6] Manthilake et al., Proc. Natl. Acad. Sci. USA, 108, pp. 17901-17904 (2011). [7] Ohta et al., Earth Planet. Sci. Lett., 349–350, pp. 109-115 (2012). [8] Okuda et al., Earth Planet. Sci. Lett., 474, pp. 25-31 (2017). [9] Hsieh et al., J. Geophys. Res., Solid Earth, 122 (7), pp. 4900-4917(2017). [10] Okuda et al., Earth Planet. Sci. Lett., Earth Planet. Sci. Lett., 520, pp. 188-198 (2019).